Solid polymer electrolyte fuel cells employ a membrane comprising a solid polymer electrolyte disposed between two porous electrically conductive electrodes, the anode and the cathode. The anode and cathode include catalysts layers, gas diffusion layers and conductive flow field plates. Fuel is fed into the anode, diffusing onto the porous catalyst layer, and is oxidized to produce electrons and protons (and CO2 as a by-product if methanol is used as the fuel). The polymer electrolyte membrane is proton conductive, in which the protons migrate towards the cathode. Oxidant, at the same time, is fed to the cathode. The oxidant diffuses onto the porous cathode catalyst layer and reacts with the protons and electrons, producing water as a by-product. The electrons travel from the anode to the cathode through an external circuit, thus producing the desired electrical power.
In stationary power applications, fuel cell systems may be required to operate continuously for a long period of time. However, in portable or traction power applications, fuel cell systems may be subjected to frequent start-up and shut down cycles. In either case, a fuel cell system must undergo a start-up process before generating the desired electrical power. On start-up, the initial system temperature will be below the normal operating temperature of the fuel cell system, and in some cases the initial temperature may be below the freezing point of water. Power output from the fuel cell system decreases with a decrease in temperature. Therefore, it is desirable to be able to start-up a fuel cell system at various initial ambient temperatures and to heat up the fuel cell system to a desired normal operating temperature. It is a challenge to heat up a cold fuel cell system quickly and safely in any application.
Conventional approaches for starting up a fuel cell include employing an external power source or a heater to heat the fuel cell to the desired operating temperature. However, this requires additional equipment just for start-up purposes.
U.S. Pat. No. 5,798,186 issued to Ballard Systems recognizes the advantage of being able to start-up a fuel cell at temperatures below freezing without the help of an external heating source. The patent teaches a method of heating up a hydrogen fuel cell system by connecting the fuel cell stack to an external circuit when hydrogen and oxidant are fed into the fuel cell stack. The effective resistance of the external circuit may be decreased so that the fuel cell stack is effectively short circuited for a period of time. This method leads to maximum electrochemical reaction rates, and the consequent generation of heat. However, localized hot spots in the proton conductive membrane can occur due to high current generation, particularly in a short circuit condition.
U.S. Pat. No. 6,068,941 discloses a method for starting up a hydrogen fuel cell system by adding an alcohol solution into the fuel cell cooling loop when the system is shut down. Upon start-up, air is fed into the cathode to oxidize the alcohol diffused from the cooling system. In this case, the cathode flow field plate must be made of materials with significant permeability to alcohol to allow the alcohol to diffuse to the cathode catalyst layer. This patent discloses a hydrogen fuel cell that requires a separate supply of methanol for use only during start-up. Thus, added complexity is introduced. Further, upon shut down of the fuel cell, the methanol is introduced into the coolant passages on the cathode side of the cell. The methanol must diffuse to the cathode catalyst layer through the cathode flow field plate, where it reacts with air.
PCT published application WO 00/65677 discloses a method of heating up a DMFC by feeding hydrogen into the anode and air into the cathode of the DMFC during short-circuit operation. The system is switched to methanol as the fuel once the desired operating temperature is reached. Thus, two supplies of fuel are necessary.
PCT published application WO 01/03216 discloses a method to heat up a hydrogen fuel cell stack by providing a heating element in the heating/cooling loop. Thus, a separate heating device is used to heat the fuel cell on start-up.
U.S. Pat. No. 6,329,089 discloses a fuel or oxidant starvation method in which fuel or oxidant supply is restricted so that the over-potential of fuel cell stack is increased. This over-potential is used as the heating source to heat up the fuel cell stack. Hydrogen or reformate is used as fuel. Again, in this method the fuel cell stack is connected to an external circuit during the heat up period. When the fuel is starved, local high temperature conditions may occur due to non-uniform current distribution. When oxidant is starved, fuel cell voltage reversal may occur.
In all these previously disclosed methods, fuel cell stack and related systems are heated. However, these methods may either cause localized hot spots in the proton conductive membrane and its potential safety issues or make the fuel cell system more complex because additional equipment and systems are needed.
Therefore there is a need for a simple and safe method to heat up a direct methanol fuel cell system.